Fluorine doped carbon coating of LiFePO4 as a cathode material for lithium-ion batteries

Chemical Engineering Journal 379 (2020) 122371

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Fluorine doped carbon coating of LiFePO4 as a cathode material for lithiumion batteries

T

⁎

Xufeng Wanga,c, Zhijun Fenga, , Xiaolong Houa, Lingling Liub, Min Hea, Xiaoshu Hea, ⁎ Juntong Huanga, Zhenhai Wenb,c, a

School of Material Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China c CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Fuzhou, Fujian 350002, China b

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

concept of fluorine doped carbon • The is proposed; fluoride is used as • Polyvinylidene carbon and fluoride source; optimal LFP@F-doped carbon • The nanocomposites delivered a practical capacity and cyclability.

A R T I C LE I N FO

A B S T R A C T

Keywords: Fluorine doped carbon LiFePO4 Polyvinylidene fluoride Cathode material Lithium-ion batteries

The present work presents an insightful study on the effect of fluorine doped carbon (FC) modification on the electrochemical performance of LiFePO4 cathode material. To this end, polyvinylidene fluoride is used as fluoride source to synthesize FC, which is designed to coat on LiFePO4 surface with formation of LiFePO4@FC nanocomposites. The microstructure and electrochemical properties of the nanocomposites are systematically examined by various characterization techniques, revealing that FC is tightly attached on surface of LiFePO4 particles forming a three dimensional (3D) conductive network structure. Such favorable structure provides advantages of good grain-to-grain electrical contact, shortening the Li+ diffusion distance between the grain interfaces, and facilitating the rapid transfer of electrons during charge–discharge. The optimal LiFePO4@FC nanocomposites, i.e., with 97.2 wt% of LiFePO4, are verified to show highly desirable electrochemical performance with superior rate capability and excellent cycling performance as the cathode material of lithium-ion batteries.

⁎

Corresponding authors at: School of Material Science and Engineering, Nanchang Hangkong University, Nanchang 330063, China (Z. Feng) and CAS Key Laboratory of Design and Assembly of Functional Nanostructures, Fujian Provincial Key Laboratory of Nanomaterials, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian 350002, China (Z. Wen). E-mail addresses: [email protected] (Z. Feng), [email protected] (Z. Wen). https://doi.org/10.1016/j.cej.2019.122371 Received 23 March 2019; Received in revised form 22 July 2019; Accepted 28 July 2019 Available online 29 July 2019 1385-8947/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic illustration for the synthesis of the LFP@FC nanocomposites and FC.

1. Introduction

combine carbon-coating and F-doping to form CeF covalent bonds, in the expectation that is able to electrochemically store and release highdensity energy [31], as well as efficiently facilitate the transportation of electrons and ions [32–36], which helps increase the rate capability of LFP. Due to the presence of LiPF6 in the electrolyte, forming CeF covalent bonds is beneficial for the permeation of electrolyte. So far, the typical process of doping F into carbon material is implemented by fluorination using XeF2 or F2 gas to form CeF covalent bonds [31,34–39], which is expensive and extremely dangerous. Therefore, it is vital to explore a suitable precursor containing C and F that can be used for in situ doping of fluoride into the carbon materials with a simple, safe and easily operated strategy. Polyvinylidene fluoride (PVDF), a high-performance polymer with only CeH and CeF covalent bonds, is clean, innoxious, secure, lowcost, environmentally friendly and commonly applied as a binder material for LIBs [40–47]. Ju et al. [41] reported that few-layer F-doped graphene foam used as an anode material for LIBs exhibited excellent electrochemical performance using PVDF as a fluoride source, and Jin et al. [42] adopted PVDF as fluoride source to prepare fluorinated layered–layered oxide powders through a newly developed organic precipitation process, which demonstrated excellent rate performance as cathode materials of LIBs. Herein, we propose the concept of fluorine doped carbon (FC) and report the synthesis of FC coating on bare LFP (denoted as LFP@FC), using PVDF as a carbon and fluoride source along with a ball-millingassisted rheological phase method combined with a solid-state reaction

LiFePO4 (LFP), one of the most promising commercial options for lithium-ion batteries (LIBs) cathode materials, has now been extensively developed because of its high redox potential of Fe2+/Fe3+ (3.45 V vs. Li+/Li), steady structure and long cycle life [1]. Furthermore, it has the advantages of low cost, high safety, environmental benign and naturally abundant raw bulk. However, the biggest obstacle regarding to LFP cathode lies in its poor electronic conductivity (10−9 to 10−10 S cm−1) [2] and weak Li+ diffusion coefficient (10−14 to 10−16 cm2 s−1) [3], which limit its applications of high-rate performance. Generally, there are three strategies to overcome this obstacle: (i) optimizing the particle size; (ii) doping with foreign atoms—to date, cation doping has got more attentions than anion doping and has been widely studied. However, anion doping, such as N [4–8], S [9,10], B [5,11], Cl [12,13] and F [14,15], is also very important for the substantial improvements of electrochemical performance of LFP. Compared with other anions, the F atom has a higher electronegativity (4.0) than N (3.0), S (2.5), B (2.0) and Cl (3.0). Moreover, F-doping will accelerate the decrease of the interfacial resistance of battery [16]. Therefore, F-doping is expected to show great potential for enhancing the electrochemical performance of LFP. (iii) Surface modification with carbon [17–26] and conducting polymers [27–29]. In particular, carbon-coating has been considered one of the most effective and desirable strategies to enhance LFP electrical conductivity [30]. In recognition of this complementarity, efforts have been made to 2

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(BET) method and the Barrett–Joyner–Halenda (BJH) model using nitrogen adsorption and desorption isotherms on a micromeritics instrument corporation sorption analyzer (Micromeritics TriStar II 3020).

[48], which manifests a highly desirable performance as cathode of LIBs. 2. Experimental

2.2.2. Electrochemical measurements To prepare the LFP and the three LFP@FC cathodes of LIBs, active materials, Super-P and PVDF binder were dispersed in N-methyl-2pyrrolidone at a weight ratio of 80:15:5 and ground for 2 h. The viscous slurry was dropped on aluminum foil and dried in a vacuum oven at 80 °C overnight to obtain an active material loading (without deducting the mass of carbon) between 1.0 and 1.5 mg cm−2. Then, CR2032 cointype half-battery were fabricated in an argon-filled glove box with pure lithium foil as the anode, Celgard 2400 (polypropylene) as the separator, and a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (a volume ratio of EC: DMC = 1:1) with 1 M of LiPF6 dissolved in as the electrolyte. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) were conducted on an electrochemical workstation (CHI660E, Shanghai Chenhua Co. Ltd., China). CV measurements were carried out at a scanning rate of 0.1–2.0 mV s−1, EIS was performed by using an AC voltage with 5 mV amplitude within a frequency range from 10−2 to 105 Hz. The galvanostatic charge–discharge was tested with a multichannel battery testing system (LAND CT2001A, Wuhan Jinnuo Electronics Co. Ltd., China) in a fixed voltage window between 2.0 and 4.3 V (vs. Li+/Li) at ambient temperature.

2.1. Material preparation 2.1.1. Preparation of raw materials PVDF was purchased from Arkema Co. Ltd.; glucose, acetone, Li2CO3, FeC2O4·2H2O and (NH4)2HPO4 were obtained from Sigmae Aldrich Co. Ltd. (Shanghai, China) without further purification. 2.1.2. Preparation of the LFP and the three LFP@FC precursors The LFP@FC nanocomposites and FC were synthesized as described below and shown in Fig. 1. As shown in the schematic illustration of the synthesis FC, PVDF pyrolysis always accompany with the appearance of some structure defects, such as: incorporations of C–F covalent bonds, topological defects, vacancies and noncyclized structures [41]. According to previous work [31,41,49–51], topological defects, vacancies and noncyclized structures together with the mesoporous structure mainly are beneficial to lithium storage, which can improve rate performance of LFP. The multiple synergistic effect of these structure defects, high surface area, and mesoporous structure in the FC not only can act as the reservoirs for lithium storage, but also accelerate the transportation Li+ and the electrons throughout the electrode matrix, resulting in the improved electrochemical properties of the LFP@FC nanocomposites. Appropriate amounts of Li2CO3, FeC2O4·2H2O, NH4H2PO4 and PVDF (the weight ratios of PVDF to LFP were 0, 2, 5 and 10 wt%, respectively) dispersed in acetone were treated under ball-milling for 6 h. The molar ratio of Fe, P and Li was 1:1:1 in the raw materials. The redundant acetone was removed from the precursors under a flowing Ar at 40 °C for 10 h. Then, the obtained precursors were dried in the vacuum oven at 80 °C overnight to further get rid of the remaining acetone. Finally, the heat treated precursors were ground into powder, and the size of powder was controlled by sieving with 500-mesh sieves.

3. Results and discussion 3.1. Characterization Fig. 2a shows the XRD patterns of the set of the LFP@FC-II and other control samples, which points that all characteristic peaks can be indexed to a primarily olivine-type structure with the orthorhombic Pnma space group. Table S1† displays the lattice parameters of the four samples derived from XRD data. These results confirm that the crystal structures of the four samples are stable. Furthermore, no crystalline carbon peaks corresponding to the residual carbon from PVDF pyrolysis can be observed in the XRD patterns of the three LFP@FC samples, suggesting that carbon content may be little or the generated carbon is in amorphous form [52]. Fig. S1† depicts the TGA curves of the LFP@ FC-II and other control samples. Due to the existence of carbonate and oxalate in the prepared precursors, the increase in the LFP sample is obviously below the theoretical increase of 5 wt%, which can be verified by Raman spectroscopy. According to a computing method in our previous work [48], the content of FC coated on LFP, for LFP@FC-I, LFP@FC-II and LFP@FC-III, is approximately 1.1, 2.8 and 5.5 wt%, respectively. Table S5† shows carbon content of the LFP and the three LFP@FC samples using a carbon-sulfur analyzer. The Raman spectrum shown in Fig. 2b indicates the degree of graphitization of carbon in the LFP@FC-II and other control samples. The D-band at ~1350 cm−1 and the G-band at ~1587 cm−1 correspond to the disordered carbon and the ordered graphitic carbon, respectively. Compared with bare LFP (ID/IG = 0.99), both of LFP@FC-I and LFP@ FC-II show a lower ID/IG ratio of 0.94 and 0.89 owing to the presence of FC as with multilayer graphene [48], suggesting that the degree of graphitization in two samples is higher. This would be in favor of the increase of the electronic conductivity, in good agreement with previous reports [53]. The ID/IG ratio of LFP@FC-III was calculated to be 1.03, which demonstrates that the superfluous doping of F will reduce the ordering degree of carbon [15]. XPS analysis was conducted to further confirm the surface elemental composition and valence states of the LFP@FC-II nanocomposites (Fig. 2). Six peaks, at 55.8, 133.5, 191.1, 284.8, 531.2 and 710.9 eV, are corresponding to Li1s, P2p, P2s, C1s, O1s and Fe2p, respectively (Fig. 2c). However, the peak of F1s, a very weak peak, can be observed owing to the relatively small doping amount of fluorine and noise

2.1.3. Synthesis of the LFP and the three LFP@FC nanocomposites Firstly, the grounded precursors were sintered at 350 °C with ramping rate of 1 °C min−1 for 2 h, and then calcinated at 600 °C with ramping rate of 5 °C min−1 for 8 h under an Ar atmosphere in a tubular furnace. After high-temperature treatment, four samples were obtained which corresponded to the different weight ratios of PVDF to LFP of 0, 2, 5 and 10 wt% (denoted as LFP, LFP@FC-I, LFP@FC-II and LFP@FCIII). Appropriate amount of PVDF powder was carbonized at 600 °C ramping rate of 5 °C min−1 for 2 h under an Ar atmosphere in a tubular furnace. 2.2. Analysis instruments 2.2.1. Materials characterization X-ray diffraction (XRD; Miniflex 600, Rigaku) was used for identifying the crystal structure using Cu Kα radiation of 0.15406 nm at a scan rate of 0.05° s−1 in the 2θ angular (10°–80°). Raman spectra were recorded on a Renishaw inVia Raman Microscope using a 532 nm laser excitation. Chemical compositions were analysed using the X-ray photoelectron spectroscopy (XPS; Shimadza-Kratos Analytical, Axis Ultra DLD) technique. Thermogravimetric analysis (TGA) was performed on a PerkinElmer TGA4000 thermogravimetric analyzer from 30 °C to 850 °C with a ramping rate of 5 °C min−1 under dry air with the flow rate of 10 mL min−1. The Morphologies of the four samples were characterized with a field emission scanning electron microscope (FESEM; FEI SU8010, Japan) coupled with an energy-dispersive X-ray spectrometer (EDS) and high-resolution transmission electron microscopy (HRTEM; JEM-2100F, Japan). The specific surface area and the pore size distribution of all simples were analysed by the Brunauer–Emmett–Teller 3

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LFP

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0 740 735 730 725 720 715 710 705 297 Binding Energy (eV)

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Fig. 2. (a) XRD patterns of the LFP, the three LFP@FC and LiFePO4 (JCPDS: 81–1173); (b) Raman spectra of the LFP@FC-II and other control samples; XPS spectra of the LFP@FC-II: (c) full spectrum (inset shows high-resolution spectra of F1s); (d) narrow spectrum of Fe2p; (e) core-level XPS spectrum of C1s.

the LFP@FC-II has a preferred crystal orientation along the (1 1 2) facet. As shown in the HRTEM image of LFP@FC-II, a carbon shell of ~2.5 nm in thickness, which is closely attached on the nanoparticles, which are in the range of 100–200 nm. The interplanar spacing of 3.557 Å corresponds to the (1 1 1) crystal plane of the orthorhombic olivine-type LFP (Fig. 3e and f). The elemental mapping results (Fig. 3g) obtained from high-angle annular dark-field scanning TEM images verify that the elements of Fe, P and O showed uniform distribution. Also, F is distributed with the same background shape as in Fig. 3g, indicating that F heteroatoms are successfully doped in carbon layers. The EDS of TEM results further reveal that LFP@FC-II nanocomposites contain Fe, P, O, C and a trace amount of F in Fig. 3h. The microstructure and surface properties of the LFP@FC-II nanocomposites are provided in Fig. 4. FESEM images indicated that the asprepared LFP and LFP@FC-II nanoparticles show quasi-olivine and quasi-spherical morphological features. As shown in Fig. 4a, bare LFP shows a terrible aggregation phenomenon and an uneven size distribution ranging from 200 to 600 nm. After being coated by FC, the LFP@FC-II shows a uniform size distribution ranged mostly between 100 and 200 nm (Fig. 4b and c). The EDS of FESEM results shown in Fig. 4d further confirmed that the LFP@FC-II nanocomposites contain Fe, P, O, C and a trace amount of F. The peak of Al observed in Fig. 4d is attributed to the aluminum foil used as a sample carrier for SEM measurement. As shown in Fig. 4e, the LFP@FC-II exhibits a characteristic type IV isotherm with a hysteresis loop [57], suggesting the presence massive mesoporous resulting from the stacking of nanoparticles [48,58]. The specific surface area of LFP@FC-II nanocomposites is 34.5 m2 g−1, which is about 5 times larger than that of the LFP (6.4 m2 g−1), revealing that the FC could dramatically enhance the surface area of LFP@FC-II, likely owing to the high specific surface area of FC (see Fig. S3†), as well as the presence of a small amount of FC could further

disruption [15]. As shown in the inset of Fig. 2c, the binding energy for F1s was measured at 687.5 eV, which fully matches the F1s peak of FC (Fig. S2†), indicating that fluorine is successfully doped into carbon molecular skeletons. A Fe2p doublet (Fe2p3/2 and Fe2p1/2) is observed that corresponds to the Fe(II) valence state, which is in good agreement with the characteristic of LFP (Fig. 2d). In addition, the C1s XPS spectrum of the LFP@FC-II nanocomposites can be deconvoluted into four peaks at 284.8, 286.3, 288.8 and 290.5 eV, attributing to CeC, CeOeC, OeC]O and CeF, respectively, in accordance with the C1s peak of FC (Figs. 2d and S2†), which indicates existence of fluorine covalently bonded to the amorphous carbon layer. It has been confirmed that fluorine-doped carbon can be effectively achieved using PVDF as a fluorine and carbon source. The doping-caused defects can increase electrochemical reactivity and electrical conductivity of LFP to a higher extent [8], due to the higher electronegativity of fluorine than carbon [54,55]. The “core-shell” structure of the LFP@FC-II nanoparticles is confirmed by TEM. Fig. 3a displays the bare LFP primary particles, where an obvious grain boundary between nanoparticles can be observed. From the HRTEM image of the LFP (Fig. 3b), the surface of LFP nanoparticles is coated with a very thin amorphous carbon layer, in accordance with the above XRD and Raman results. And the lattice fringe with a space of 2.502 Å agrees well with the (3 1 1) crystal plane of orthorhombic olivine-type LFP. As revealed in Fig. 3c, a uniformly amorphous carbon coating layer can be obviously seen on the surface of the LFP particle, forming a “core-shell” conducting network structure that could effectively impede the further growth of LFP nanoparticles [56]. Moreover, some irregular pores can also be clearly seen between nanoparticles which can increase the specific surface area of the LFP@ FC-II nanocomposites and are beneficial for the full penetration of electrolyte and the rapid transport of ion. The selected area electron diffraction (SAED) pattern of LFP@FC-II (inset of Fig. 3d) indicates that 4

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2 4 6 8 Binding Energy (keV)

Cu 10

Fig. 3. TEM images of (a) the LFP and (c, d, e) the LFP@FC-II. HRTEM image of (b) the LFP and (f) the LFP@FC-II corresponding to the yellow-boxed area in (a) and (e), respectively; (g) TEM image and element mapping images of the LFP@FC-II; (h) EDS image of the LFP@FC-II corresponding to the red-cross dot in (g); The inset of (d) shows the SAED of the LFP@FC-II. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the voltage range from 2.0 to 4.3 V. One can see that every sample has a typical potential plateau at ~3.45 V associated with the reverse reaction of Fe3+ to Fe2+ [48]. As is seen from the embedded Fig. 5a, the charge–discharge voltage plateau and gap of the LFP@FC-II is more stable and narrower than that of other control samples, suggesting that the LFP@FC-II can provide the more expedite paths for the migration of Li+ [53,59] and suffer from a lower polarization loss than do other control samples (especially the LFP sample). The result highlights the decreases the polarization resistance of the LFP@FC-II that could result from FC coating, forming a “core-shell” conducting network structure that could effectively inhibit the further growth of LFP nanoparticles and shorten the Li+ diffusion distance, as well as facilitate the rapid migration of electrons in the Fe3+/Fe2+ redox reaction. Fig. 5b displays the comparison of the rate capabilities of the LFP@ FC-II and other control samples at different rate. The LFP@FC-II exhibits a high discharge capacity (174.3 mA h g−1) at 0.1 C, which exceeds the theoretical capacity and is much higher than that of other

decrease the size of LFP nanoparticles, leading to LFP nanoparticles in LFP@FC-II smaller than bare LFP nanoparticles. Hence, the “core-shell” structure of LFP@FC-II is able to shorten the diffusion distance of Li+ and preserve the possibility of diffusion into the LFP nanoparticles from three dimensions, resulting in a smaller polarization to enhance the electrochemical performance of LFP [59]. Based on the BJH model, the pore size distribution of the LFP@FC-II centered at ~4 nm (Fig. 4f). With the increase in the PVDF powder additive amount, the BET surface area of LFP@FC nanocomposites gradually increases (see Figs. 4e and S3†).

3.2. Electrochemical performance In order to examine the role of FC in affecting the electrochemical performance of LFP, the electrochemical tests of the LFP and the three LFP@FC cathode materials are evaluated using coin cells. Fig. 5a presents the typical charge–discharge profiles of four samples at 0.2 C in 5

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(b)

Intensity (a.u.)

O

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Fig. 4. FESEM images of (a) the LFP and (b, c) the LFP@FC-II; (d) EDS image of the LFP@FC-II corresponding to the red-cross dot; (e) nitrogen adsorption–desorption isotherms of the LFP and the LFP@FC-II; (f) pore size distribution of the LFP@FC-II. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

4.5

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Fig. 5. (a) Charge–discharge profiles of the LFP@FC-II and other control samples at 0.2 C (inset shows part of the flat region magnified); (b) Charge–discharge profiles of the LFP@FC-II and other control samples in the potential region from 2.0 to 4.3 V at various rates; (c) Rate performance (herein referred to as discharge capacity) of the LFP@FC-II and other control samples at various rates; (d) Cycle performance of the LFP@FC-II and other control samples at 10 C (herein referred to as discharge capacity). 6

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Fe3+. However, the potential interval of the two redox peaks and current intensity of the peaks are distinctly different from each other. First, LFP@FC-II presents the lowest potential interval of 163 mV between the two redox peaks, whereas that of LFP, LFP@FC-III and LFP@FC-I is 593, 217 and 191 mV, respectively. Second, a large increase can also be seen between the corresponding peak currents of LFP and LFP@FC and the order of peak current (Ip) is LFP@FC-II > LFP@FC-I > LFP@FCIII > LFP. In contrast, the two redox peaks of LFP@FC-II are more symmetric, sharper and narrower relative to other samples, indicating a better electrochemical reactivity and lower polarization of the electrode, more optimized electrode kinetics and reversibility [48], which is consistent with the charge–discharge test results. This is attributed to FC coating on the bare LFP that could enhance the redox kinetics and decrease the polarization of the electrode in the LFP@FC-II. Fig. 6b and c exhibit CV curves of the LFP@FC-II and other control samples at various scanning rates. The symmetric and sharp redox peaks indicate excellent reversibility for the three LFP@FC samples, especially for the LFP@FC-II, standing in stark contrast to the redox peaks of the LFP that have inferior symmetry and the oxidation peaks are indistinguishable (Fig. 6b). Moreover, with the increasing scanning current density, the polarization phenomenon is more severe, indicating that the bare LFP shows poor electrochemical performance. Fig. S4† exhibits a linear dependency between the peak current and the square root of the scanning rate (ν). According to a computing method in our previous work [48], the average of Li-ion diffusion coefficients of the three LFP@FC samples during charge–discharge processes are summarized in Table S2†. This study finds that the content of FC coating plays a critical role in the enhancement of electrochemical performance of LFP@FC. The results indicate that the addition of 5 wt% PVDF is optimum for the diffusion transport of Li+ in the three LFP@FC samples. This is likely because the addition of 5 wt% PVDF is more suitable for the formation of graphene-like amorphous carbon layers, forming a 3D “core-shell” network structure with LFP nanoparticles and providing a channel for the diffusion transport of Li+, which can contribute to enhance rate capabilities of LFP. To analyze the cause of the dramatically increased rate capability, EIS studies were conducted on the LFP@FC-II and other control samples. The fitted Nyquist plots are shown in Fig. 6d, which is comprised of three regions:the ohmic resistance (Rs) in the intersection at high frequency with the Z′ axis, corresponding to the resistance of the electrolyte, is in the Nyquist plots [63]; the double layer capacity (Q) and the charge transfer resistance (Rct) in the medium frequency region of the semicircle, corresponding to the electrochemical reaction at the electrode–electrolyte interface and particle–particle contact [57]; the Warburg resistance (W) in the low-frequency region, associating with the diffusion of Li+ in the LFP nanoparticles [61]. The Warburg coefficient (σ) is related to –Z″ (−Z″ = σ × ω−1/2; ω denotes frequency) [48]. Table S3† shows the fitting results of the LFP@FC-II and other control samples, the Rct (299.55 Ω) value of the LFP electrode is the largest of the four samples, indicating that FC play a vital role in the improvement of electron transfer kinetics. By examining the three LFP@FC samples, the order of Rct is found to be LFP@FC-III > LFP@ FC-I > LFP@FC-II. This result indicates that the LFP@FC-II undergoes a slight polarization phenomenon during the cycling process owing to the optimum content of FC. This not only could effectively balance the proportion of active materials and FC to decrease the polarization resistance of the electrode but also could greatly improve the electron transfer kinetics of the LFP@FC-II nanocomposites during Li+ insertion–extraction. For LFP@FC-II, σ (10.31) is much lower than that of other control samples, indicating that the optimum content of FC helps the LFP@FC-II obtain a high rate capability (Fig. 6e) [27,48].

control samples. In the FC, the presence of defects could offer some redox active sites for lithium storage. Due to the unique structure of FC, FC incorporated with the LFP could enhance the rate capabilities of bare LFP from FC and this storage mechanism is reversible [60]. Hence, it is reasonable that the discharge capacity of LFP@FC-II surpassed the theoretical values for LiFePO4 at 0.1 C. Compared with other LFP@FC electrodes, the LFP@FC-II exhibits discharge capacities of about 159.3 and 121.5 mA h g−1 at 1 and 10 C, respectively, while the LFP@FC-I shows 144.8 and 100.1 mA h g−1 and the LFP@FC-III electrode only shows 135.1 and 92.5 mA h g−1 at the corresponding current rates. Even at 20 C, the LFP@FC-II could still maintain 58.9% of the theoretical capacity, whereas the LFP@FC-I and the LFP@FC-III only show 46.1% and 36.1% of the theoretical capacity, respectively. These results indicate that the LFP@FC-II can provide the higher energy density and power density than other control samples [53]. The different rate discharge capacities of the LFP@FC-II and other control samples are depicted in the Fig. 5c. Obviously, the LFP@FC-II electrode exhibits the highest discharge capacity in all electrodes at the same rate (in particular at high rates), suggesting that appropriate amount of FC in situ coating the LFP nanoparticle architecture is beneficial for tolerant to varied charge and discharge currents [8]. Along with the increase of the current rate, the capacity decay of the three LFP@FC electrodes is very small. However, with the further increase of the current rate, the discharge capacities are dropping owing to the bigger interfacial resistance of the electrode and the sluggish Li+ diffusion kinetics at high current rates [48]. These results demonstrate that appropriate amount of FC could uniformly wrap the entire surface of LFP nanoparticles, leading to sufficient rate capabilities of composites. Less carbon content may does not cover the entire LFP surface, leading to nonuniform coating or unsatisfactory surface conductivity forming an insufficient electronically conducting network [8]. However, more FC content would reduce the content of the electroactive materials on the current collector, resulting in inferior rate capabilities in the LFP@FC-III nanocomposites. Meanwhile, more FC content also could impact the well contact between electrolyte and active materials, causing the restriction of the Li+ rapid diffusion [61]. The above test results confirm that the LFP@FC-II exhibits the best rate capabilities among the three LFP@FC samples at the same current rate owing to the optimum content of FC in the LFP@ FC-II, which not only could balance the proportion of active materials and FC to decrease the polarization resistance of the electrode but also help the diffusion and transport of electrolyte ion, providing the substantial improvement for the rate performance of LFP. The cyclic stability of the LFP@FC-II and other control samples is investigated at 10 C in Fig. 5d. The LFP@FC-II shows an initial discharge capacity of 122.6 mA h g−1, corresponding to 72.1% of the theoretical capacity, and after 1000 cycles, the retention rate of reversible capacity is 86.6%. The LFP@FC-I and LFP@FC-III hold 69.7% and 71.5% capacity retention on the same testing conditions after 1000 cycles’ charge–discharge, respectively. The capacity fading may be caused by the irreversible reaction between the electrolyte and active materials [53]. However, for the bare LFP, the capacity decay is 4.6% after 1000 cycles. This suggests that the structure of LFP is very stable and not destroyed after 1000 cycles. These results reveal that the LFP@ FC-II nanocomposites present a better cycling stability than that of other control samples, which was attributed to the fast capacity response of the “core-shell” conductive network constructed with FC and LFP nanoparticles. The structure can not only accommodate the volume change of the electrode for releasing the strain on or from LFP [62], but also buffer polarization increment during Li+ insertion–extraction processes. CV is considered as a useful testing technique to study the phase transformation and the conductivities of ionic and electronic. Fig. 6a shows CV plots of the LFP@FC-II and other control samples at a scan rate of 0.1 mV s−1. One can observe a pair of anodic-cathodic peaks, accompanying with the redox reaction conversion between Fe2+ and

4. Conclusions In this study, we prepared a set of three conductive network 7

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0.6

163 mV

LFP LFP@FC-I LFP@FC-II LFP@FC-III

0.3 0.0

217 mV

191 mV

-0.3 593 mV

-0.6 2.0

2.5

3.5

(b) LFP

0.1 0.0

0.1mV

-0.1

0.2mV 0.5mV

-0.2

1 mV 2 mV

-0.3 2.0

4.0

2.5

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3.5

+

Voltage (V vs Li /Li)

(c)

3

4.0

0.1mV 0.2mV

2

0.5mV

1

1 mV 2 mV

-3 2.0

2.5

3 .0

3 .5

4.0

2 .0

2 .5

LFP LFP@FC-I LFP@FC-II LFP@FC-III

3500

3.0

(e) 800

60

2 .0

2 .5

3 .0

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4 .0

LFP LFP@FC-I LFP@FC-II LFP@FC-III

600

2500 30

2000

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+

1000

(d)

90

3000

3.5

Voltage (V vs Li /Li)

120

4000

LFP@FC-III

-2

LFP@FC-I

LFP@FC-II

0

-1

-Z'' (ohm)

+

Voltage (V vs Li /Li)

4

Current (mA)

3.0

0.2

-Z'' (

Current (mA)

0.3

(a)

Current (mA)

0.9

400

1500

0

30

60

90

120

150

0 180

1000

Rct

200

W

Re

500 0

0

Q

0

300

600

Z' (ohm)

900

1200

2.0

2.5

3.0 -1/2

3.5

4.0

Fig. 6. (a) CV plots of the LFP@FC-II and other control samples at a scan rate of 0.1 mV s−1; (b) and (c) CV plots of the LFP@FC-II and other control samples at different scan rates, respectively; (d) Nyquist and equivalent circuit plots of the LFP@FC-II and other control samples; (e) Fitting curves of –Z″ and ω−1/2 at a lowfrequency region.

Declaration of Competing Interest

structure composites with FC coating LiFePO4. The experimental results show that approximately 2.8 wt% content of FC is the optimum content. The hybrid composites with 2.8 wt% FC decorating (LFP@FC-II) exhibit significantly improved performance, including a high capacity approaching to the theoretical value, an attractive cycling stability over 1000 cycles, and an impressively high rate capability with maintaining 100.2 mA h g−1 at 20 C. The present work demonstrates the strategy with FC coating LFP could open a new avenue to maximize the performance for LFP cathode materials in prospect as a high-performance cathode material for commercial LIBs.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments This work was supported by the National Natural Science Foundations of China (Grant No. 51302131 and Grant No. 51772140) and the Aeronautical Science Foundation of China (2015ZF56017). 8

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Appendix A. Supplementary data [27]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2019.122371.

[28]

References

[29]

[1] A.K. Padhi, K.S. Nanjundaswamy, J.B. Goodenough, Phospho-olivines as positiveelectrode materials for rechargeable lithium batteries, J. Electrochem. Soc. 144 (1997) 1188–1194. [2] Y. Huang, J.B. Goodenough, High-rate LiFePO4 lithium rechargeable battery promoted by electrochemically active polymers, Chem. Mater. 20 (2008) 7237–7241. [3] J. Wang, X. Sun, Understanding and recent development of carbon coating on LiFePO4 cathode materials for lithium-ion batteries, Energy Environ. Sci. 5 (2012) 5163–5185. [4] J. Yang, J. Wang, X. Li, D. Wang, J. Liu, G. Liang, M. Gauthier, Y. Li, D. Geng, R. Li, X. Sun, Hierarchically porous LiFePO4/nitrogen-doped carbon nanotubes composite as a cathode for lithium ion batteries, J. Mater. Chem. 22 (2012) 7537–7543. [5] J. Zhang, N. Nie, Y. Liu, J. Wang, F. Yu, J. Gu, Wei Li, Boron and nitrogen co-doped carbon layers of LiFePO4 improve the high-rate electrochemical performance for lithium ion batteries, ACS Appl. Mater. Inter. 7 (2015) 20134–20143. [6] Y. Liu, J. Gu, J. Zhang, F. Yu, L. Dong, N. Nie, W. Li, Metal organic frameworks derived porous lithium iron phosphate with continuous nitrogen-doped carbon networks for lithium ion batteries, J. Power Sources 304 (2016) 42–50. [7] Y. Zhang, Y. Huang, X. Wang, Y. Guo, D. Jia, X. Tang, Improved electrochemical performance of lithium iron phosphate in situ coated with hierarchical porous nitrogen-doped graphene-like membrane, J. Power Sources 305 (2016) 122–127. [8] P. Wang, G. Zhang, Z. Li, W. Sheng, Y. Zhang, J. Gu, X. Zheng, F. Cao, Improved electrochemical performance of LiFePO4@N-doped carbon nanocomposites using polybenzoxazine as nitrogen and carbon sources, ACS Appl. Mater. Inter. 8 (2016) 26908–26915. [9] S.B. Lee, S.H. Cho, V. Aravindan, H.S. Kim, Y.S. Lee, Improved cycle performance of sulfur-doped LiFePO4 material at high temperatures, B. Korean Chem. Soc. 30 (2009) 2223–2226. [10] D. Xu, P. Wang, B. Shen, Synthesis and characterization of sulfur-doped carbon decorated LiFePO4 nanocomposite as high performance cathode material for lithium-ion batteries, Ceram. Int. 42 (2016) 5331–5338. [11] J. Feng, Y. Wang, High-rate and Ultralong Cycle-life LiFePO4 Nanocrystals coated by boron-doped carbon as positive electrode for lithium-ion batteries, Appl. Surf. Sci. 390 (2016) 481–488. [12] C. Sun, Y. Zhang, X. Zhang, Z. Zhou, Structural and electrochemical properties of Cldoped LiFePO4/C, J. Power Sources 195 (2010) 3680–3683. [13] F. Pan, W. Wang, Synthesis and characterization of core-shell F-doped LiFePO4/C composite for lithium-ion batteries, J. Solid State Electr. 16 (2012) 1423–1427. [14] Z. Wang, L. Yuan, M. Wu, D. Sun, Y. Huang, Effects of Na+ and Cl- co-doping on electrochemical performance in LiFePO4/C, Electrochim. Acta 56 (2011) 8477–8483. [15] F. Lu, Y. Zhou, J. Liu, Y. Pan, Enhancement of F-doping on the electrochemical behavior of carbon-coated LiFePO4 nanoparticles prepared by hydrothermal route, Electrochim. Acta 56 (2011) 8833–8838. [16] Y. Li, H. Xu, P.H. Chien, N. Wu, S. Xin, L. Xue, K. Park, Y.Y. Hu, J.B. Goodenough, A perovskite electrolyte that is stable in moist air for lithium-ion batteries, Angew. Chem. Int. Ed. 57 (2018) 8587–8591. [17] J. Wang, J. Yang, Y. Tang, J. Liu, Y. Zhang, G. Liang, M. Gauthier, Y.C. Karen, C. Wiegart, M.N. Banis, X. Li, R. Li, J. Wang, T.K. Sham, X. Sun, Size-dependent surface phase change of lithium iron phosphate during carbon coating, Nat. Commun. 5 (2014) 3415. [18] A. Varzi, C. Ramirez-Castro, A. Balducci, S. Passerini, Performance and kinetics of LiFePO4-carbon bi-material electrodes for hybrid devices: a comparative study between activated carbon and multi-walled carbon nanotubes, J. Power Sources 273 (2015) 1016–1022. [19] W. Deng, S. Ci, H. Li, Z. Wen, One-step ultrasonic spray route for rapid preparation of hollow Fe3O4/C microspheres anode for lithium-ion batteries, Chem. Eng. J. 330 (2017) 995–1001. [20] C.W. Ahn, J.J. Choi, J. Ryu, B.D. Hahn, J.W. Kim, W.H. Yoon, J.H. Choi, D.S. Park, Microstructure and electrochemical properties of graphite and C-coated LiFePO4 films fabricated by aerosol deposition method for Li ion battery, Carbon 82 (2015) 135–142. [21] G. Du, Y. Zhou, X. Tian, G. Wu, Y. Xi, S. Zhao, High-performance 3D directional porous LiFePO4/C materials synthesized by freeze casting, Appl. Surf. Sci. 453 (2018) 493–501. [22] X. Zhao, Y. Bi, S.Y. Choe, S.Y. Kim, An integrated reduced order model considering degradation effects for LiFePO4/graphite cells, Electrochim. Acta 280 (2018) 41–54. [23] D. Li, D.L. Danilov, B. Zwikirsch, M. Fichtner, Y. Yang, R.A. Eichel, P.H.L. Notten, Modeling the degradation mechanisms of C6/LiFePO4 batteries, J. Power Sources 375 (2018) 106–117. [24] S.W. Oh, S.T. Myung, S.M. Oh, K.H. Oh, K. Amine, B. Scrosati, Y.K. Sun, Double carbon coating of LiFePO4 as high rate electrode for rechargeable lithium batteries, Adv. Mater. 22 (2010) 4842–4845. [25] G.T.K. Fey, Y.C. Lin, H.M. Kao, Characterization and electrochemical properties of high tap-density LiFePO4/C cathode materials by a combination of carbothermal reduction and molten salt methods, Electrochim. Acta 80 (2012) 41–49. [26] Y. Wu, Z. Wen, J. Li, Hierarchical carbon-coated LiFePO4 nanoplate microspheres

[30]

[31] [32] [33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

9

with high electrochemical performance for Li-ion batteries, Adv. Mater. 23 (2011) 1126–1129. Y. Shi, X. Zhou, J. Zhang, A.M. Bruck, A.C. Bond, A.C. Marschilok, K.J. Takeuchi, E.S. Takeuchi, G. Yu, Nanostructured conductive polymer gels as a general framework material to improve electrochemical performance of cathode materials in Liion batteries, Nano Lett. 17 (2017) 1906–1914. K.S. Park, S.B. Schougaard, J.B. Goodenough, Conducting-polymer/iron-redoxcouple composite cathodes for lithium secondary batteries, Adv. Mater. 19 (2007) 848–851. G. Xie, C. Ma, X. Zhang, H. Liu, L. Yang, L. Yang, K. Wang, Y. Wei, Chitosan-based cross-linked fluorescent polymer containing aggregation-induced emission fluorogen for cell imaging, Dyes Pigments 143 (2017) 276–283. T. Liu, L. Zhao, J. Zhu, B. Wang, C. Guo, D. Wang, The composite electrode of LiFePO4 cathode materials modified with exfoliated graphene from expanded graphite for high power Li-ion batteries, J. Mater. Chem. A 2 (2014) 2822–2829. W. Feng, P. Long, Y. Feng, Y. Li, Two-dimensional fluorinated graphene: synthesis, structures, properties and applications, Adv. Sci. 3 (2016) 1500413. H. Cong, S. Xin, S. Yu, Flexible nitrogen-doped graphene/SnO2 foams promise kinetically stable lithium storage, Nano Energy 13 (2015) 482–490. L. Chen, L. Song, Y. Zhang, P. Wang, Z. Xiao, Y. Guo, F. Cao, Nitrogen and sulfur codoped reduced graphene oxide as a general platform for rapid and sensitive fluorescent detection of biological species, ACS Appl. Mater. Inter. 8 (2016) 11255–11261. F. Cao, M. Zhao, Y. Yu, B. Chen, Y. Huang, J. Yang, X. Cao, Q. Lu, X. Zhang, Z. Zhang, C. Tan, H. Zhang, Synthesis of two-dimensional CoS1.097/nitrogen-doped carbon nanocomposites using metal-organic framework nanosheets as precursors for supercapacitor application, J. Am. Chem. Soc. 138 (2016) 6924–6927. Y. Xu, X. Zhu, X. Zhou, X. Liu, Y. Liu, Z. Dai, J. Bao, Ge nanoparticles encapsulated in nitrogen-doped reduced graphene oxide as an advanced anode material for lithium-ion batteries, J. Phys. Chem. C 118 (2014) 28502–28508. S. Lou, X. Cheng, Y. Zhao, A. Lushington, J. Gao, Q. Li, P. Zuo, B. Wang, Y. Gao, Y. Ma, C. Du, G. Yin, X. Sun, Superior performance of ordered macroporous TiNb2O7 anodes for lithium ion batteries: understanding from the structural and pseudocapacitive insights on achieving high rate capability, Nano Energy 34 (2017) 15–25. J.T. Robinson, J.S. Burgess, C.E. Junkermeier, S.C. Badescu, T.L. Reinecke, F.K. Perkins, M.K. Zalalutdniov, J.W. Baldwin, J.C. Culbertson, P.E. Sheehan, E.S. Snow, Properties of fluorinated graphene films, Nano Lett. 10 (2010) 3001–3005. K.J. Jeon, Z. Lee, E. Pollak, L. Moreschini, A. Bostwick, C.M. Park, R. Mendelsberg, V. Radmilovic, R. Kostecki, T.J. Richardson, E. Rotenberg, Fluorographene: a wide bandgap semiconductor with ultraviolet luminescence, ACS Nano 5 (2011) 1042–1046. K.I. Ho, J.H. Liao, C. Huang, C.L. Hsu, W. Zhang, A. Lu, L. Li, C. Lai, C. Su, One-step formation of a single atomic-layer transistor by the selective fluorination of a graphene film, Small 10 (2014) 989–997. J. Luo, C. Fang, N. Wu, High polarity poly(vinylidene difluoride) thin coating for dendrite-free and high-performance lithium metal anodes, Adv. Energy Mater. 8 (2018) 1701482. Z. Ju, S. Zhang, Z. Xing, Q. Zhuang, Y. Qiang, Y. Qian, Directly synthesis few-layer F-doped graphene foam and its lithium/potassium storage properties, ACS Appl. Mater. Inter. 8 (2016) 20682–20690. Y.C. Jin, J.G. Duh, Fluorination induced the surface segregation of high voltage spinel on lithium-rich layered cathodes for enhanced rate capability in lithium ion batteries, ACS Appl. Mater. Inter. 8 (2016) 3883–3891. D. Gryzlov, S. Novikova, T. Kulova, A. Skundin, A. Yaroslavtsev, Behavior of LiFePO4/CPVDF/Ag-based cathode materials obtained using polyvinylidene fluoride as the carbon source, Mater. Des. 104 (2016) 95–101. R.R. Kapaev, S.A. Novikova, A.A. Chekannikov, D.Y. Gryzlov, T.L. Kulova, A.M. Skundin, A.B. Yaroslavtsev, Effect of carbon sources and synthesis conditions on the LiFePO4/C cathode properties, Rev. Adv. Mater. Sci. 57 (2018) 183–192. I.A. Stenina, T.L. Kulova, A.M. Skundin, A.B. Yaroslavtsev, Effects of carbon coating from sucrose and PVDF on electrochemical performance of Li4Ti5O12/C composites in different potential ranges, J. Solid State Electr. 22 (2018) 2631–2639. L. Li, C. Fan, T. Zeng, X. Zhang, W. Zhang, S. Han, Electrochemical performances of Li3V2-(4/3)xTix(PO4)3/C as cathode material for Li-ion batteries synthesized by an ultrasound-assisted sol-gel method, J. Alloys Compd. 650 (2015) 136–142. L. Li, C. Fan, X. Zhang, T. Zeng, W. Zhang, S. Han, Synthesis of Li3V2(PO4)3/C for use as the cathode material in lithium ion batteries using polyvinylidene fluoride as the source of carbon, New J. Chem. 39 (2015) 2627–2632. X. Wang, Z. Feng, J. Huang, W. Deng, X. Li, H. Zhang, Z. Wen, Graphene-decorated carbon-coated LiFePO4 nanospheres as a high-performance cathode material for lithium-ion batteries, Carbon 127 (2018) 149–157. C. Sun, Y. Feng, Y. Li, C. Qin, Q. Zhang, W. Feng, Solvothermally exfoliated fluorographene for high-performance lithium primary batteries, Nanoscale 6 (2014) 2634–2641. C. Zhang, N. Mahmood, H. Yin, F. Liu, Y. Hou, Synthesis of phosphorus-doped graphene and its multifunctional applications for oxygen reduction reaction and lithium ion batteries, Adv. Mater. 25 (2013) 4932–4937. L. Qie, W. Chen, Z. Wang, Q. Shao, X. Li, L. Yuan, X. Hu, W. Zhang, Y. Huang, Nitrogen-doped porous carbon nanofiber webs as anodes for lithium ion batteries with a superhigh capacity and rate capability, Adv. Mater. 24 (2012) 2047–2050. Z. Wu, W. Ren, L. Xu, F. Li, H. Cheng, Doped graphene sheets as anode materials with superhigh rate and large capacity for lithium ion batteries, ACS Nano 5 (2011) 5463–5471. B. Wang, W.A. Abdulla, D. Wang, X. Zhao, A three-dimensional porous LiFePO4

Chemical Engineering Journal 379 (2020) 122371

X. Wang, et al.

[54]

[55]

[56]

[57]

[58]

[59] B. Wang, B. Xu, T. Liu, P. Liu, C. Guo, S. Wang, Q. Wang, Z. Xiong, D. Wang, X. Zhao, Mesoporous carbon-coated LiFePO4 nanocrystals co-modified with graphene and Mg2+ doping as superior cathode materials for lithium ion batteries, Nanoscale 6 (2014) 986–995. [60] L.H. Hu, F.Y. Wu, C.T. Lin, A.N. Khlobystov, L.J. Li, Graphene-modified LiFePO4 cathode for lithium ion battery beyond theoretical capacity, Nat. Commun. 4 (2013) 1687. [61] Q. Zhang, S. Huang, J. Jin, J. Liu, Y. Li, H. Wang, L. Chen, B. Wang, B. Su, Engineering 3D bicontinuous hierarchically macro-mesoporous LiFePO4/C nanocomposite for lithium storage with high rate capability and long cycle stability, Sci. Rep. 6 (2016) 25942. [62] W. Wei, S. Yang, H. Zhou, I. Lieberwirth, X. Feng, K. Mullen, 3D graphene foams cross-linked with pre-encapsulated Fe3O4 nanospheres for enhanced lithium storage, Adv. Mater. 25 (2013) 2909–2914. [63] A. Fedorková, R. Orináková, A. Orinák, M. Kupková, H.D. Wiemhöfer, J.N. Audinot, J. Guillot, Electrochemical and XPS study of LiFePO4 cathode nanocomposite with PPy/PEG conductive network, Solid State Sci. 14 (2012) 1238–1243.

cathode material modified with a nitrogen-doped graphene aerogel for high-power lithium ion batteries, Energy Environ. Sci. 8 (2015) 869–875. K. Zhang, J.T. Lee, P. Li, B. Kang, J.H. Kim, G.R. Yi, J.H. Park, Conformal coating strategy comprising N-doped carbon and conventional graphene for achieving ultrahigh power and cyclability of LiFePO4, Nano Lett. 15 (2015) 6756–6763. Y. Zhang, Y. You, S. Xin, Y. Yin, J. Zhang, P. Wang, X. Zheng, F. Cao, Y. Guo, Rice husk-derived hierarchical silicon/nitrogen-doped carbon/carbon nanotube spheres as low-cost and high-capacity anodes for lithium-ion batteries, Nano Energy 25 (2016) 120–127. B. Wang, D. Wang, Q. Wang, T. Liu, C. Guo, X. Zhao, Improvement of the electrochemical performance of carbon-coated LiFePO4 modified with reduced graphene oxide, J. Mater. Chem. A 1 (2013) 135–144. R. Chen, Y. Wu, X. Kong, Monodisperse porous LiFePO4/C microspheres derived by microwave-assisted hydrothermal process combined with carbothermal reduction for high power lithium-ion batteries, J. Power Sources 258 (2014) 246–252. S. Lou, X. Cheng, J. Gao, Q. Li, L. Wang, Y. Cao, Y. Ma, P. Zuo, Y. Gao, C. Du, H. Huo, G. Yin, Pseudocapacitive Li+ intercalation in porous Ti2Nb10O29 nanospheres enables ultra-fast lithium storage, Energy Storage Mater. 11 (2018) 57–66.

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